This invention relates to a process for making thermally stable polymers, including graft copolymers comprising a backbone of a propylene polymer material.
All polymers, natural or synthetic, generally undergo thermal degradation at high temperatures, both in the presence and in the absence of oxygen. The degree of degradation varies for different polymers and can arise not only from the wide variety of chemical structures, but for a given polymer, from differences in crystallinity, morphology, or the nature and amount of catalyst residue that is present. Degradation of the polymer results not only in increasing melt flow index or color, but also affects the melt processability of the polymers at elevated temperatures and their mechanical properties over time.
One type of polymer that is particularly susceptible to thermal degradation at relatively low temperatures is polymers that are substituted at the alpha-carbon atom such as poly(methacrylates), poly(methacrylonitriles) and poly(alpha-methylstyrene). Of these, poly(methacrylates) and particularly poly(methyl methacrylate) (PMMA) are the most widely used in commercial applications. However, their use is limited by their poor thermal stability at processing temperatures. Copolymerization with a minor amount of non-methacrylate monomer such as methyl acrylate, ethyl acrylate, butyl acrylate or styrene is often required to improve stability for processing in commercial applications. If a comonomer is not used, PMMA could depolymerize readily at high conversion ( greater than 95%) above 300xc2x0 C. However, copolymerization affects the mechanical and molecular properties of PMMA. Depolymerization during processing also raises safety and health concerns due to monomer exposure, and odor associated with the monomer. The free monomer in the polymer can also lead to surface irregularities called splay, as well as odor in the finished product.
U.S. Pat. No. 6,046,273 describes a method for increasing the thermal stability of alpha-substituted acrylate graft copolymers by grafting to the backbone of propylene polymer material monomers comprising (a) an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group and (b) an ester of an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group. International Patent Publication WO 00/08078 describes a method for increasing the thermal stability of acrylic-grafted propylene polymers by graft polymerizing in the presence of a 4-vinyl-substituted 5-12 C cyclic 1-alkene, which acts as an end-capping agent.
Polymers that are unsubstituted in the alpha-position such as polyethylene, polypropylene, polystyrene, and polyacrylonitrile, generally degrade by random scission at temperatures higher than those at which depolymerization of alpha-substituted polymers takes place, resulting in rapid loss of molecular weight and mechanical properties. In the presence of oxygen, degradation can also proceed via formation of hydroperoxide species, causing further decomposition.
U.S. Pat. No. 5,914,194 discloses vinylidene chloride compositions having enhanced thermal stability and a decreased tendency to degrade while being extruded by using certain fatty acid derivatives, particularly fatty acid derivatives of castor oil, as a stabilizer in amounts up to about 4.0% by weight, along with up to about 2 weight percent of a plasticizer. Use of at least one aliphatic unsaturated compound, e.g., soybean oil or safflower oil, to stabilize polyolefin compositions against radiation-induced degradation is disclosed in U.S. Pat. No. 6,017,986.
There is still a need for a method to increase the thermal stability of polymers, including graft copolymers of propylene polymer materials, under melt processing conditions, compared with that achievable with methods currently used for this purpose.
The process of this invention for improving the thermal stability of polymers comprises blending, before melt processing, (1) a polymer selected from the group consisting of (a) olefin polymers, (b) polymers of vinyl-substituted aromatic compounds, (c) polymers of acrylic esters selected from the group consisting of (i) an ester of an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group, and (ii) a combination of (i) and at least one monomer capable of being polymerized by free radicals, and (d) graft copolymers comprising a backbone of a propylene polymer material having graft polymerized thereto a monomer selected from the group consisting of (i) an ester of an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group, and (ii) a combination of (i) and at least one monomer capable of being polymerized by free radicals, and (2) about 0.1% to about 5%, based on the weight of the polymer, of at least one aliphatic compound having at least one site of unsaturation, the compound having a molecular weight of at least 200 and an iodine number of at least 10.
Also disclosed is a composition comprising (1) a polymer selected from the group consisting of (a) polymers of vinyl-substituted aromatic compounds, (b) polymers of acrylic esters selected from the group consisting of (i) an ester of an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group, and (ii) a combination of (i) and at least one monomer capable of being polymerized by free radicals, and (c) graft copolymers comprising a backbone of a propylene polymer material having graft polymerized thereto a monomer selected from the group consisting of (i) an ester of an acrylic acid substituted at the alpha-carbon atom by a 1-3 C alkyl group, and (ii) a combination of (i) and at least one monomer capable of being polymerized by free radicals, and (2) about 0.1% to about 5%, based on the weight of the polymer, of at least one aliphatic compound having at least one site of unsaturation, the compound having a molecular weight of at least 200 and an iodine number of at least 10.
The aliphatic unsaturated compound retards degradation of the polymer during melt processing. The improvement in thermal stability of the polymer is indicated by a lower melt flow rate (MFR), a shifting of the weight loss vs temperature curve generated by thermogravimetric analysis (TGA), towards higher temperatures, and/or lowering of the amount of the residual monomer in pelletized samples.